The invention relates to a method for producing a switching signal on the passage of a contrast jump between two fields having a contrast movable relative to a photoelectric receiving arrangement having at least two adjacent photodetectors through a predetermined point with reference to the receiving arrangement, whereby the outputs of the photodetector are combined to form the switching signal.
For the control or regulation of the automatic operating sequence various production processes, particularly in the paper-processing industry require optically functioning scanners to detect printed colour marks. In this connection, the printed colour mark is moved at a corresponding speed through the visual range of the mark scanner and a signal is emitted via optical sensors. As the quality of the products produced, e.g. in the case of multicolour printing, is inter alia dependent on the precise local detection of the printing mark, the maximum divergence of the switching point location of the mark scanner must be less than 0.005mm from a fixed reference location. No displacement in the switching point location must be caused by colour changes in the printing mark and reflectance changes of the mark carrier. Wherever possible, there should be no adjusting device for the optimum adaptations to the particular contrast conditions which exist between background and printing mark.
In known mark scanners, optical means focus the light of a filament lamp and project onto the material surface to be scanned. Part of the light reflected by the material surface is passed via further optical means to a photodetector and is converted into an electrical quantity. If the electrical quantity exceeds or drops below a predetermined switching threshold, a switching signal is emitted, whose appearance is a measure of the switching point location.
However, this known method has the disadvantage that firstly the switching threshold must be adapted to the particular contrast difference, because e.g. a strongly reflecting mark carrier gives rise to a larger signal than the switching threshold and the weakly reflecting printing mark gives rise to a signal which is smaller than the switching threshold, the opposite being the case when using bright marks on a dark background. Secondly, due to the finite extension of the scanning light beam when the printing mark enters a constantly rising or falling luminous flux change is obtained which at a specific value exceeds or drops below the switching threshold.
In the case of changes to the reflection factor of the printing mark or mark carrier and in the case of changes to the transmitting light flux rising above or dropping below the switching threshold take place at different penetration stages of the mark into the scanning light beam, so that correspondingly different switching point locations are obtained, which is undesirable.
In a further known method (published German Patent Application No. 2,152,732), three juxtaposed photodetectors are used on which is produced a photographic image of the illuminated printing mark. Evaluation takes place in such a way that a switching signal is emitted when a differential signal occurs between the first and third photodetectors and the sum of the photoelectric current of the first and third elements has half the value of the photoelectric current of the second photodetector. This is the case when the image edge of the printing mark is precisely in the centre of the second element. Thus, the first photodetector sees the mark carrier and the second photodetector the printing mark.
Although this known method provides the necessary precision, makes unnecessary an adjusting device for adapting to the particular contrast difference and supplies the necessary two pulses on entering and leaving the mark image edge, due to the printing mark widths and the limits imposed by the dimensions of the scanning head used in this method, it is not always possible to project onto the triple photodetector group the necessary image size by means of the imaging lens. Thus, the geometrical value of the triple photodetector group must be adapted to the image. In practice, it has been found that corresponding triple photodetector combinations can seldom be provided true to size with other requisite data.